Prior Art FIG. 3 shows a basic, simple photovoltaic cell 40 comprising a transparent substrate 41, e.g. glass with a layer of a transparent conductive oxide (TCO) 42 deposited thereon. This layer is also called front contact and acts as first electrode for the photovoltaic element. The combination of substrate 41 and front contact 42 is also known as superstrate. The next layer 43 acts as the active photo-voltaic layer and exhibits three “sub-layers” forming a p-i-n junction. Said layer 43 comprises hydrogenated micro-crystalline, nanocrystalline or amorphous silicon or a combination thereof. Sub-layer 44 (adjacent to TCO front contact 42) is positively doped, the adjacent sub-layer 45 is intrinsic, and the final sub-layer 46 is negatively doped.
In an alternative embodiment the layer sequence p-i-n as described can be inverted to n-i-p, then layer 44 is identified as n-layer, layer 45 again as intrinsic, layer 46 as p-layer.
Finally, the cell includes a rear contact layer 47 (also called back contact) which may be made of zinc oxide, tin oxide or ITO and a reflective layer 48. Alternatively a metallic back contact may be realized, which can combine the physical properties of back reflector 48 and back contact 47. For illustrative purposes, arrows indicate impinging light. Solar cells, also known as photovoltaic cells, are semiconductor devices that convert electromagnetic energy, such as light or solar radiation, directly into electricity. The semiconductors used in the light absorbing layers of such cells are characterized by energy band gaps between their valence electron bands and their conduction electron bands (“midgap-material”). Free electrons usually cannot exist or remain in these band gaps.
When light is absorbed by said semiconductor material electrons in low-energy states (valence band) can be excited and jump the band gap to unoccupied higher energy states in the conduction band. Electrons excited to higher energy states leave behind unoccupied low-energy positions which are referred to as holes. These holes may shift from atom to atom in the semiconductor matrix and thus act as charge carriers in the valence band as do free electrons in the conduction band and thus contribute to conductivity. Most of the photons that are absorbed in the semiconductor create such electron-hole pairs. These electron-hole pairs generate photocurrent and, in the presence of a built-in field, the photovoltage of the solar cells.
Electron hole pairs produced by the light would eventually recombine, and convert to heat or a photon, unless prevented from doing so. To prevent recombination, a local electric field is created in the semiconductor by doping or interfacing dissimilar materials to produce a space charge layer. The space charge layer separates the holes and electrons for use as charge carriers. Once separated, these collected hole and electron charge carriers produce a space charge that results in a voltage across the junction, which is the photovoltage. If these separated hole and charge carriers are allowed to flow through an external load, they would constitute a photocurrent. Technically this is realized by establishing a thin p- and a n-doped semiconductor layer adjacent to the i-layer of a. m. p-i-n-structure. The i-layer or intrinsic layer exhibits the absorber behaviour for light and the generation of a. m. electron-hole pairs.
In practice, the semiconductor must be designed with a small band gap so that even photons from lower energy radiation can excite electrons to jump the band gap, but, in doing so, there are at least two negative effects that must be traded.
First, the small band gap results in a low photovoltage device, and thus low power output occurs. Secondly, the photons from higher energy radiation will produce many hot carriers with much excess energy that will be lost as heat upon immediate thermalization of these hot carriers to the edge of the conduction band.
On the other hand, if the semiconductor is designed with a larger band gap to increase the photovoltage and reduce energy loss caused by thermalization of hot carriers, then the photons from lower energy radiation will not be absorbed. Therefore, in designing conventional single junction solar cells, it is necessary to balance these considerations and try to design a semiconductor with an optimum band gap, realizing that in the balance, there has to be a significant loss of energy from both large and small energy photons.
Materials such as silicon with a band gap of 1.1 eV are relatively inexpensive and are considered to be good solar energy conversion semiconductors for conventional single junction solar cells.
The conversion efficiency of a solar cell is determined by the current of the cell, the voltage and the fill factor. The capacity of a cell to absorb light or the absorption coefficient of an intrinsic absorber layer determines the maximum density of current which can be achieved. It is very important to choose an absorber material which can absorb as much as possible of the incident light. Only photons with energy higher than the band gap of the absorber layer can be absorbed. The energy at which charge carriers are extracted from the cell determines the voltage of the cell. The fill factor is a quality factor which also indicates the fraction of carriers which recombine in the solar cell. The higher the fill factor, the higher is the quality of the absorber layer. If there are fewer defects in the absorber layer, there will be less centers of recombination, hence there will be less loss due to recombination.
In crystalline silicon the tetrahedral structure of a silicon atom with coordination number 4 is continued over a large range. In amorphous silicon the atoms form a continuous random network; therefore no long range order is present. This material has a disordered nature; not all atoms have a coordination number of 4. It appears that some defects, called dangling bonds, act as recombination centers and result from weak or broken bond of Si—H. These dangling bonds are responsible for the anomalous electrical behavior. To overcome the disorder, the material can be passivated by hydrogen. Hydrogenated amorphous silicon (a-Si:H) has a low amount of such defects, but unfortunately (a-Si:H) is associated with the Staebler-Wronsky Effect or light induced degradation (LID).
It has been shown that diluting the well known precursor gas silane with hydrogen has a beneficial effect on the quality of a-Si:H layers. It improves the order of the structure which is again beneficial for the stability of the optoelectronic material based on a-Si:H.
For layers deposited by PECVD, it has been shown that the hydrogen dilution decreases the growth rate for all known deposition techniques such as RF, DC, VHF. A growth rate of 4 Å/s has been achieved for films with a good stability. The films with best stability however have been deposited at a comparably lower growth rate of 2 Å/s.
A state-of-the-art deposition process results in an intrinsic layer deposited with a process gas dilution ratio of 1 (silane:hydrogen=1:1). Good efficiency results have been achieved with a deposition rate of 3.6 Å/s. Even higher deposition rates can be achieved by increasing the RF power during the deposition, but the layers' quality and uniformity decrease. The common advantage of thin film a-Si:H based solar cells is that they can been fabricated at low cost, however the deposition rate is comparably low.
The challenge is therefore to have a high quality intrinsic absorber layer (with few defects) and a suitable band gap. For decreasing the costs of solar cells a high throughput of the deposition systems is a key factor, since the absorber layer occupies the major part of any p-i-n solar cell structure. Hence the deposition rate of the absorber layer is very important.
For thin film solar cells it is important to achieve both high throughput and as much as possible high quality material. It helps decreasing the production cost and provides for a relative high efficiency. The known technologies face the problem of throughput and stability. The stability of devices can be improved by increasing the hydrogen dilution in the i-layer, but then the throughput is decreased. If the focus is laid on the throughput, the uniformity of the i-layer on a big surface (1.4 m2 or more) suffers, because this last parameter is worsening when the power is increased for obtaining high rates.
It is one object of the present invention to improve throughput for and simultaneously maintaining quality of the addressed absorber layer.
Generically this is resolved by combining the pressure, the power and the gas flows to a new processing regime in such a way that one can overcome the uniformity and stability problem at an increased deposition rate.
This is achieved by a method of manufacturing an intrinsic absorber layer of amorphous hydrogenated silicon in a p-i-n configuration of a solar cell by PECVD depositing said layer upon a base structure for said layer in a reactor, said depositing comprising;                establishing in said reactor a pressure of between 1 mbar and 1.8 mbar,        establishing a flow of silane and of hydrogen with a dilution of silane to hydrogen of 1:4 up to 1:10        generating a RF Plasma with a generator power of between 600 W and 1200 W per 1.4 m2 base structure surface to be coated.        
In one embodiment of the method according to the invention the addressed depositing is performed at a temperature of 200° C.
In one embodiment of the method according to the invention depositing is performed at a growth rate of 4.4 Å/s to 6.6 Å/s.
In one embodiment of the method according to the invention there is established one of:                the pressure: 1 mbar and        the flow of silane: 450 sccm per 1.4 m2 base structure surface        the flow of hydrogen: 2000 sccm per 1.4 m2 base structure surface        the power: 600 W per 1.4 m2 base structure surface or, preferably        the pressure: 1.8 mbar and        the flow of silane: 450 sccm per 1.4 m2 base structure surface        the flow of hydrogen: 2000 sccm per 1.4 m2 base structure surface        the power: 1200 W per 1.4 m2 base structure surface.        
It is further an object of the invention to improve the stability of the intrinsic absorber layer. It is a known phenomenon that boron from the p-layer can diffuse into the adjacent i-layer, which is detrimental to its light absorption properties.
So as to additionally resolve this object in a further embodiment of the invention wherein there is valid:                the surface of the base structure is the surface of a p-layer of the p-i-n cell and there is provided a buffer layer between the addressed surface of said p-layer and said absorber layer        or        the surface of the base structure is the surface of a n-layer of the p-i-n cell and there is provided a buffer layer between the addressed absorber layer and a subsequent layer to be deposited upon said absorber layer,        depositing of the buffer layer of amorphous hydrogenated silicon is performed by an equal deposition process as deposition of the absorber layer with the following differing settings:                    the addressed pressure: ½ to ⅓ of the pressure established for depositing the absorber layer,            the addressed flow of silane: ½ to ¼ of the flow of silane established for depositing the absorber layer,            the addressed flow of hydrogen: 2-2.5 times the flow of hydrogen established for depositing the absorber layer,            the addressed power: ½ to ⅓ of the power applied for depositing the absorber layer.                        
In the embodiment addressed last, there is further preferably added to the reactor space, for depositing said buffer layer, CH4 gas.
In a further embodiment the addressed adding of CH4 is performed at a flow of ⅔ of the flow of silane4.
In a further embodiment of the invention the absorber layer is deposited with a thickness of between 100 nm and 600 nm, preferably of between 150 nm and 300 nm.
Still in a further embodiment having a buffer layer the buffer layer is deposited with a thickness of between 5 nm to 15 nm, preferably of between 7 nm and 11 nm.
In a further embodiment of the invention having a buffer layer deposited, the ratio of deposition rate for the buffer layer to deposition rate for the absorber layer is selected to be between 1:7 and 1:5.
It is also an object of the invention to improve the stability of the intrinsic absorber layer. It is a known phenomenon that boron from the p-layer can diffuse into the adjacent i-layer, which is detrimental to its light absorption properties.
This object is resolved by a method of manufacturing a buffer layer of amorphous hydrogenated silicon between an intrinsic absorber layer of amorphous hydrogenated silicon in a p-i-n solar cell and the p-layer of such p-i-n solar cell, by PECVD depositing said buffer layer in a reactor on a base structure comprising                establishing in the reactor a pressure of between 0.3 mbar and 0.9 mbar,        establishing in the reactor a flow of silane between 180 sccm and 225 sccm per 1.4 m2 of base structure surface to be coated,        establishing in the reactor a flow of hydrogen of between 4000 sccm and 5000 sccm per 1.4 m2 of base structure surface to be coated,        generating an RF Plasma with a generator power of between 200 W and 600 W per 1.4 m2 base structure surface to be coated.        
In an embodiment of the just addressed method there is added to the reactor space for depositing said buffer layer CH4 gas.
In a further embodiment such adding CH4 gas is performed at a flow of ⅔ of the flow of silane.